1. Field of the Invention
The present invention relates generally to the field of nonvolatile memory devices for use in computers and other devices. More particularly, the present invention relates to nonvolatile memory arrays that use magnetic tunnel junction memory elements as individual memory cells.
2. Description of the Related Art
Certain types of magnetic memory cells that use the magnetic state of a ferromagnetic (FM) region for altering the electrical resistance of materials located near the ferromagnetic region are collectively known as magnetoresistive (MR) memory cells. An array of magnetic memory cells is often called a magnetic random access memory (MRAM).
In comparison to metallic MR memory cells, which are based on giant magnetoresistance (GMR) or anisotropic magnetoresistance (AMR) devices, MRAM memory cells are based on magnetic tunnel junction (MTJ) devices and rely on substantially different physical principles. For example, GMR devices include at least two ferromagnetic layers that are separated by a thin metallic layer. In contrast, an MTJ device has two ferromagnetic layers that are separated by a thin insulating tunnel barrier. The magnetoresistance of an MTJ device results from a spin-polarized tunneling of conduction electrons between the two ferromagnetic layers that depends on the relative orientation of the magnetic moments of the two ferromagnetic layers. Another important distinction between GMR and MTJ devices is that current flows parallel to the thin film layers forming a GMR device, whereas current flows perpendicularly to the thin film layers forming an MTJ device.
It is possible to form GMR memory cells in which the current passes perpendicularly to the thin film layers forming the GMR device for reading and possibly writing the cell. In such a device, the magnetic moment of the free layer can be rotated or can be aided to rotate by the self-field of the current flowing through the device itself. See, for example, J. M. Daughton, Magnetoresistive memory technology, Thin Solid Films 216, 162, 1992. By using current passing through the GMR memory cell, in addition to currents passing through word and/or bit line, the magnetic switching of a selected GMR cell in a GMR memory cell array is improved. Such a memory cell, however, unless extremely small, will have such a low resistance that there will be extremely little signal available for reading the device without a correspondingly high read current.
U.S. Pat. No. 5,695,864 to J. Slonczewski discloses a physical mechanism for switching the magnetization of FM layers by passing a current through a GMR or an MTJ element. According to Slonczewski, electrical transport of spins exerts a magnetic force on the magnetic moment of the film, thereby making rotation of the magnetic moment, in theory, possible. This effect has recently been demonstrated in exotically prepared tiny metallic GMR devices by J. A. Katine et al., Current-driven magnetization reversal and spin wave excitation in Co/Cu/Co pillars, Phys. Rev. Lett. (to be published in 2000), and in very narrow metallic GMR wires formed by electro-deposition in etched ion beam tracks in polymer membranes by J. E. Wegrowe et al., Euro. Phys. Lett. 45,626 (1999). Nevertheless, this effect has not been demonstrated in MTJs because the current densities must be much higher than are presently provided in MTJs.
MRAM designs have also been disclosed having various read select mechanisms. For example, U.S. Pat. No. 5,734,605 to Zhu et al. discloses a read select mechanism that uses a transistor for each cell. U.S. Pat. No. 5,640,343 to Gallagher et al. discloses a read select mechanism that uses a series diode for each cell. U.S. patent application Ser. No. 09/549,172, relates to a read select mechanism that uses a series non-linear tunnel junction for each cell, and U.S. patent application Ser. No. 09/549,212, relates to a read select mechanism that uses the intrinsic non-linearity of the MTJ itself.
U.S. Pat. No. 5,734,605 to Zhu et al. and U.S. Pat. No. 5,640,343 to Gallagher et al. both disclose that writing is performed by a cross-selection geometry in which a vector sum of the fields generated by write currents flowing in two orthogonal bit and word lines switches the magnetization of a selected cell located at the cross point of the bit and word lines. The vector sum of the field at the cross point is 2 larger than the field generated by the bit or word line alone, which provides a way for selecting an individual magnetic cell.
FIG. 1A shows an MRAM array formed from magnetic tunneling junction (MTJ) memory cells, such as disclosed in U.S. Pat. No 5,640,343. The array includes a first set of substantially parallel electrically conductive lines that function as parallel word lines 1-3 and a second set of substantially parallel electrically conductive lines that function as parallel bit lines 4-6. Word lines 1-3 are formed in a first horizontal plane and bit lines 4-6 are formed in a second horizontal plane. Bit lines 4-6 are oriented in a direction that is generally perpendicular to word lines 1-3. Bit lines 4-6 are preferably oriented at right angles from word lines 1-3, so that the two sets of lines appear to intersect, or overlap, when viewed from above. A plurality of intersection regions is defined between the plurality of overlaps of the two sets of lines. An MTJ memory cell 9 is located at each intersection region between the intersecting lines. While only three word lines and six bit lines are shown in FIG. 1A, the number of word and bit lines would typically be much larger.
MTJ memory cell 9 is arranged in a vertical stack and includes a selection device 7, such as a silicon junction diode, and an MTJ device 8. Selection device 7 includes an n-type silicon layer 10 and a p-type silicon layer 11. Layer 10 is connected to word line 3. Layer 11 is connected to MTJ element 8 via a tungsten stud 12.
MTJ device 8 is formed from a series of layers of material stacked one on top of the other. A template layer 13, such as Pt, is formed on stud 12. An initial ferromagnetic layer 14, such as permalloy (Ni--Fe), is formed on template layer 13. An antiferromagnetic layer (AF) 15, such as Mn--Fe, is formed on layer 14. A fixed ferromagnetic layer (FMF) 16, such as Co--Fe or permalloy, is formed on layer 15. A thin tunneling barrier layer 17 of alumina (Al.sub.2 O.sub.3) is formed on layer 16. A soft ferromagnetic layer (FMS) 18, such as a sandwich of thin Co--Fe with permalloy, is formed on layer 17. Lastly, a contact layer 19, such as Pt, is formed on layer 18.
To read the state of a selected MTJ memory cell in the array of FIG. 1A, a current is passed through the MTJ memory cell. Various methods have been proposed to allow for read selection of a particular memory cell. For example, U.S. Pat. No. 5,640,343 to Gallagher et al. discloses that a field effect transistor 21 can be used in series with each MTJ device 22, such as shown in FIG. 2A. Gallagher et al. disclose that alternatively a diode 23 can be used in series with each MTJ device 22, such as shown in FIG. 2B. U.S. patent application Ser. No. 09/549,172, discloses that a non-magnetic tunnel junction 24 can be used in series with each MTJ 22, such as shown in FIG. 2C. U.S. patent application Ser. No. 09/549,212, discloses that a strongly non-linear MTJ device 25 alone can be used, such as shown in FIG. 2D.
Conventionally, the writing of a MTJ cell in an MTJ-MRAM array is performed by sending currents through the corresponding word and bit lines of the selected MTJ cell. Two independent currents are passed through the array along the word and bit lines. Unlike the read process, no current is designed to flow through the selected cell. A conventional writing process is depicted in each of FIGS. 2A-2D where the current paths through the array for writing a selected MTJ cell are represented by a heavy word line WI and a heavy bit line B1.
Typical MRAM memory cells do not have identical switching fields. The switching fields differ from cell to cell based on variations in memory cell shape and size, variations in thickness of the thin film layers forming each memory cell, damage to a cell introduced during fabrication, and/or different magnetic histories to which the magnetic device forming the respective cells have been subjected. Consequently, there is likely to be some variation in the switching fields of the magnetic devices in a memory array. The variations may be so large that it becomes difficult to write a selected cell without affecting the state of half-selected cells, that is, the cells along the word and bit lines of the selected cell. This problem is likely to become more severe as the size of the memory cells decreases and there is a consequent larger relative variation in shape and size of the memory cells.
The field obtained at the cross point of the selected word and bit lines is the vector sum of the self-field of the respective currents passed through the word and bit lines, which is only about 2 times larger than the field obtained solely from either of the bit or write line currents. More importantly, the field used for switching the selected MTJ device at the cross-point is only about 40% larger than the field to which the half-selected devices are subjected. Thus, the margin for write selectivity is small, and is reduced by any variation in switching fields of the individual MTJ devices within a given memory array.
The switching field of an MTJ device is sensitive to, for example, the exact shape and roughness of the edges of the MTJ device. The switching field is also influenced by the thickness of both of the ferromagnetic electrodes, as well as the thickness of the tunnel junction. The field may also be influenced by the processing used to form the MTJ device. For example, some of the magnetic material removed during the patterning of an MTJ device may be re-deposited within the MTJ array. For these reasons, it is extremely difficult to prepare arrays of magnetic memory cells having extremely homogeneous magnetic switching fields. Nevertheless, unless the MTJ devices of an array can be formed to have homogeneous magnetic switching fields, MTJ devices other than the selected device may switch during the writing of the selected cross-point memory cell, especially the MTJ cells located along the selected bit and write lines.
In view of the foregoing, what is needed is a way for improving the write selectivity of individual MTJ memory cells in an MRAM array over the write selectivity obtained by a vector combination of the self-fields of currents passing through orthogonal word and bit lines.